Automatic test equipment, commonly referred to as a semiconductor tester, provides a critical role in the manufacture of semiconductor devices. The equipment enables the functional test of each device both at the wafer stage and the packaged-device stage. By verifying device operability and performance on a mass-production scale, device manufacturers are able to command premium prices for quality products.
One conventional type of automatic test system includes a computer-driven test controller and a test head connected electrically to the controller by a heavy-duty multi-cable. A manipulator mechanically carries the test head. The test head generally includes a plurality of channel cards that mount the pin electronics necessary to generate the test signals or patterns to each I/O pin or contact of one or more devices-under-test (DUTs).
One of the primary purposes of the test head is to place the channel card pin electronics as close to the DUT as practicable to minimize the distance that signals must propagate therebetween. The length and construction of the signal path interfacing the test head to the DUT, commonly referred to as a tester interface, directly affects signal delays and signal losses. Consequently, tester interface schemes that interconnect the pin electronics to the DUT play an important role in the achievable accuracy of a semiconductor tester.
With reference to FIG. 1, one conventional high performance tester interface includes a connector module 12 that houses the terminations for a plurality of coaxial cables 14. The signal conductor (not shown) for each cable couples to a compliant spring-biased contact, more commonly known as a pogo pin 16, while each cable shield couples to the signal pogo barrel. The signal pogo barrel connects, in a side-stepped fashion, to the module 12 as a ground connection. A ground pogo pin assembly 18 connects to the signal pogo barrel to continue the ground path through to a device-interface-board (DIB, not shown). Typically, a plurality of ground paths surround each signal path to minimize high frequency interference.
While the conventional pogo-based tester interface described above works well for its intended applications, one of the drawbacks is a practical bandwidth barrier of around 1 GHz. At such high frequencies, the signal path characteristics emulate transmission lines, generally requiring matched 50-ohm environments. Deviations from the 50-ohms often cause signal degradations that lead to timing inaccuracies and the like. Inaccuracy in the tester may improperly fail devices that perform near threshold levels.
Conventional interface signal path constructions, such as that described above, generally employ numerous connections and discontinuities that affect the characteristic impedance. These constructions often cause reflections at high frequencies that substantially degrade signals at frequencies around 1 GHz. Consequently, for high speed and high accuracy testing of semiconductor devices at and above the 1 GHz range, conventional pogo-pin interface schemes are disfavored.
Conventional pogo pins also present density problems for high channel count testers. For instance, it's generally recognized that to test each pin of a 1024-pin semiconductor device, the tester should have at least 1024 channels (one channel for each pin). Such a high number of signal channels also requires ground and power connections, often resulting in over six-thousand connections for interfacing between the tester and the DUT. With a typical center-to-center spacing of around 0.150 inches, the achievable density or “pitch” of six thousand conventional pogo pins would require an undesirably large DIB. This is unacceptable to many semiconductor device manufacturers that require very efficient “footprints” to maximize available clean-room space. Moreover, this would also require long traces on the DIB to route signals to and from the DUT.
One proposal for a tester interface that avoids the use of conventional pogo pins is disclosed in U.S. Pat. No. 5,944,548 to Saito. The patent discloses a floating coaxial connector scheme that employs an intermediately disposed mount member formed with an oversized opening. The opening receives a spring member and biases a female connector for mating with a male connector. The opening is formed to allow for slight pivoting of the mated coaxial connectors, allegedly minimizing the difficulty in making a standard connection.
While this construction appears beneficial for its intended application, the implementation of relatively large coaxial connectors for each tester channel at the probecard end fails to address the problems noted above regarding channel density and overall tester footprint size.
What is needed and heretofore unavailable is a tester interface that avoids the use of conventional pogo pins and provides high bandwidth signal performance while maximizing tester channel density. These capabilities in turn are believed to minimize the costs attributable to semiconductor device testing. The tester interface module of the present invention satisfies these needs.